1. Field of the Invention
The invention relates to charge pump circuits with a flying capacitor, and more particularly to the use of a flying capacitor which is charged as a function of the output voltage error.
2. Description of the Related Art
Portable electronic devices such as cellular phones, personal digital assistants, portable MP3 player and laptops are invariably powered by batteries which may be rechargeable or non-rechargeable. All of these batteries lose their charge over time, and therefore do not produce a single constant voltage output. One device for providing a regulated voltage supply independent of the battery output voltage is a charge pump. Charge pumps are DC-to-DC converters which utilize capacitors instead of inductors or transformers for energy storage. Such charge pumps typically make use of a “flying capacitor” which is charged from the battery during one cycle and pumped up, thereby raising its output voltage, during another cycle. The charge stored during the charging cycle is then transferred into a storage capacitor during the pump cycle.
Related art regulated charge pumps charge the flying capacitor up to the input voltage producing excessive overshoot and ripple at the output terminal of the charge pump circuit. These regulated charge pumps regulate the output voltage by changing the switching frequency. A big output capacitor is therefore required to reduce the overshoot and ripple of the output voltage because the entire excess charge of the flying capacitor will be transferred to the output capacitor in each switching period. The excess is the charge which causes the voltage of the flying capacitor to be higher than the output voltage minus the input voltage.
Related Art U.S. Patents are described next:    U.S. Pat. No. 6,801,078 (Allum) discloses a voltage multiplier circuit for programmable memories. It further utilizes a voltage feedback regulator which provides a digital gating signal to a multiplexer.    U.S. Pat. No. 6,794,927 (Bedarida et al.) teaches the use of a regulated supply voltage with a modular arrangement of charge pumps. The desired output voltages are obtained by using combinatorial clock signals for charge pump stages.    U.S. Pat. No. 6,794,926 (Rader et al.) shows a charge pump comprising an input circuit, a switching circuit and a control circuit which receives its inputs from the switching circuit and input circuit, and controls the switching circuit. The switches of the switching circuit are selectively actuated to charge and discharge capacitors to provide a regulated output voltage.    U.S. Pat. No. 6,774,710 (Li) describes a high precision charge pump circuit which compares the output feedback voltage with a reference voltage. In response to the comparison the clock frequency is either increased or decreased.    U.S. Pat. No. 6,756,838 (Wu et al.) presents a charge pump voltage regulator involving a load current and a clamp current. The two currents are compared and the charge pump current is then adjusted by stepping the frequency of the clock driving the charge pump.    U.S. Pat. No. 6,618,296 (Zhang) discloses a charge pump circuit for controlling the charging current via current limiting devices in a dual phase charge pump circuit. Supply current peaks that occur when the flying capacitor is charged are limited, thus reducing the noise on the supply.
It should be noted that none of the above-cited examples of the related art provide a current into the flying capacitor which is a function of the output voltage error, where the output voltage error is the desired (nominal) output voltage minus the actual output voltage. In addition the present invention utilizes a fixed frequency, in the first embodiment, or a very low frequency, in the second embodiment, and avoids frequencies ranging from audio frequencies to Megahertz frequencies. The present invention also limits the current to the output terminal to avoid short circuits. Additionally, the present invention provides currents of 100 mA at the output terminal using an external capacitor.